A proliferation of so-called “smart phones” has created renewed interest in cutting glass and sapphire using laser radiation. Typically a smart phone has a transparent cover plate that can be a single sheet cover plate or a multilayer cover plate. In either case, an outer sheet is typically of a chemically strengthened glass with surface regions thereof under high compressive stress. A usual shape for the sheet is a rectangle with rounded corners. A very fine finished edge is required. This provides a challenge for most accepted cutting methods because of the brittle nature of the chemically strengthened glass and a tight radius of the rounded corners, typically a few millimeters (mm). The amount of material to be cut is enormous, as such smart phones are sold in quantities of millions, with new models appearing almost annually.
Recently, a method of using pulsed lasers has been advertised as ideal for cutting chemically strengthened glass. The pulses have a wavelength of about 1064 nanometers (nm) and a duration of less than about 10 picoseconds (ps). The pulses are delivered in bursts of about 5 pulses separated in time by a few nanoseconds (ns), with bursts delivered at a repetition rate of about 200 kilohertz (kHz). The pulses are delivered in a manner which causes the formation of elongated filaments (voids) in the glass. The filaments extend almost completely through the thickness of the sheet. The glass is moved during the delivery of the pulses so that a “curtain” of such filaments is produced along cutting path. The glass can then be separated along the cutting path by applying mechanical pressure, or may separate spontaneously along the cutting path.
This process is described, in principle at least, in U.S. Pre-Grant Publication No. 2013/0126573. What is missing from the description is a description of focusing optics used for delivering the pulses to the glass. Using focusing optics generally accepted and useful for other laser machining processes, the inventors were not able to reproduce the results described in the publication, even after many months of experimentation.
Before the filing date of the '573 publication, it was known that filaments could be created in glass when drilling high aspect-ratio holes using femtosecond and picosecond laser pulses. A particularly favored method to create filaments included using focusing optics to generate an elongated focal region in the glass. These beams are generally referred to as Bessel beams by practitioners of the art.
Such Bessel beams have been produced by including a non-conventional optical element such as an axicon in the focusing optics. Bessel beams have also been generated by including a phase-mask (transparent diffractive element) in the focusing optics. Axicons and phase-masks are expensive elements to produce, primarily because they are not suited to batch (volume) production. Accordingly, without any guarantee that Bessel-beam optics were the “missing link” in the '573 document, it was decided by the inventors to devise a simpler arrangement for generating filaments in glass and other brittle materials which could be the key to reproducing the results described therein.